1. Field of the Invention
The present invention is related to light blocking and more particularly to blocking light generated at a specific wavelength corresponding to a particular color or color range that is distracting or disruptive to a driver during a normal operation by use of an inexpensive construction while avoiding blocking light in a range of wavelengths that are important for human visual acuity.
2. Description of the Related Art
Conventionally known image pickup devices can include, for example, a projector that is installed in proximity to an instrument cluster in a vehicle compartment. The projector projects light including near-infrared wavelength light towards the face of a driver. A camera similarly installed in proximity to the instrument cluster picks up reflected light obtained when the projected light is reflected by the face of the driver.
Conventional image pickup devices can be configured to detect an unstable state of the driver, such as a sleepy or inattentive state, using a sensing means for detecting characteristics of facial images such as closed eyes, and the like. Since such devices must stably pick up the image of a face any time day or night, light, including near-infrared light, can be projected from the projector to the face of the driver forming a reference light level.
Some conventional image pickup devices can be constructed as follows to prevent a human from becoming aware of being an object of image pickup. The devices can include a projector, an image pickup means for picking up images using an electromagnetic wave in a wavelength range to which humans are insensitive, and a visible light blocking means that lets though electromagnetic waves in the wavelength range to which humans are insensitive, while blocking visible light. The visible light blocking means is installed in the light path between the image pickup means and a human so that the image pickup means is invisible to the human. Some conventional image pickup devices further include an illuminating means for illuminating the object of image pickup with an electromagnetic wave in a wavelength range to which humans are insensitive to light such as is described, for example in JP-A-6-189166. A concrete example of a conventional projector near-infrared light source includes a light emitting diode (LED) and the like. It should further be noted that LEDs have not only a single wavelength but also a range of wavelengths having a certain width.
Some of these image pickup devices are enhanced in sensitivity on the long wavelength side through use of an image pickup device having high sensitivity to a near-infrared wavelength. Examples of such devices are described, for example, in JP-A-6-233306, JP-A-2001-018717, and JP-A-2001-194161.
Since the amount of light of one LED is usually insufficient to favorably pick up images, some of the conventional projectors are so constructed that LEDs can be arranged in an array configuration on a circuit board. Thus, a brightness sufficient for a camera to pick up a desired image is obtained, and the near-infrared light is broadened so that, for example, a human face can be evenly illuminated. In such a conventional device, LEDs may be configured or caused to continuously emit light. However, “continuously” emitting LEDs are usually caused to emit light in a pulsed manner in synchronization, for example, with the shutter speed of a camera so as to lengthen the lives of the LEDs and reduce current consumption in the LED.
It should be noted however, that conventional devices, such as the image pickup device described in JP-A-6-189166 suffer limitations. For example, as described above, LEDs associated with conventional projectors have a single wavelength and a range of wavelengths having a certain width. Consequently, when an inexpensive visible light blocking means completely blocks visible wavelengths, it also blocks light in a range of visible wavelengths required for visibility. The above-mentioned visible light blocking means could be constructed of a filter that can completely block visible wavelengths, however an expensive multilayer film is required, rendering the approach economically impractical.
Long-wavelength LEDs have low sensitivity within the range in which the image pickup device is used, resulting in degraded efficiency. As descried in documents JP-A-6-233306, JP-A-2001-018717 and JP-A-2001-194161, while an image pickup device having high sensitivity in a near-infrared range can be used enhance sensitivity on the long wavelength side, such an image pickup device is expensive, rendering yet another approach economically impractical.